1. Technical Field
The present invention relates generally to imaging devices having focal plane imaging arrays and, more particularly, to a focal plane imaging array for a direct detection imaging device capable of conducting radiometric imaging at microwave or millimeter-wave frequencies.
2. Discussion
Direct detection radiometric imaging is an imaging technique that involves scanning a field of view and collecting the radiant electromagnetic energy which is reflected and/or emitted by unknown objects and converting that energy into useful image data. Direct detection imaging, which is a passive imaging technique, differs from active imaging methods, such as radar imaging, which generates pulses of radiation that are first transmitted outward toward the field of view and then received as they are reflected back by the objects in the field of view. Typically, direct detection imaging devices employ an optical system to collect the radiant electromagnetic energy emanating from the objects in the field of view, a focal plane imaging array (FPIA) to detect the energy and produce an output, additional electronics, such as an interface unit and processing unit, which converts the output of the FPIA into useable image data and, finally, an output display unit for displaying a resultant visible image.
An FPIA is generally comprised of an array of energy detecting pixel elements which are positioned at the focal plane of the optical system of the imaging device. As the imaging device scans a field of view, the energy is received and detected at each pixel of the FPIA.
Direct detection radiometric imaging at microwave or millimeter-wave frequencies possesses unique advantages which are desirable in a variety of military, scientific and commercial imaging applications, including surveillance systems, mapping and navigation systems. In particular, at these frequencies, which are relatively low in the realm of direct detection radiometric imaging, low visibility obstacles do not inhibit the ability to perform the imaging function and obtain useable image data. For example, the thermal radiation that is emitted at microwave or millimeter-wave frequencies by objects is capable of penetrating atmospheric conditions such as fog, haze, light rain, dust and smoke. Consequently, radiometric imaging at these frequencies is not impeded by such phenomena.
However, because radiant electromagnetic energy emissions at microwave or millimeter-wave frequencies are generally very weak, the FPIAs used in direct detection radiometric imaging devices for these frequencies must be extremely sensitive. Further, as these imaging devices process the received energy, they must not significantly distort the level of energy that is detected. That is, they must have extremely low inherent noise characteristics.
Presently, direct detection imaging devices performing imaging at microwave or millimeter-wave frequencies employ FPIAs that are very expensive because they are based upon labor intensive, microwave integrated circuit (MIC) technology. In addition, due in part to the costly MIC manufacturing techniques, only a few hundred pixels can be economically integrated into an FPIA, where thousands of pixels are desirables. This results in a FPIA with a severely limited field of view. Consequently, present FPIAs used for direct detection radiometric imaging at microwave or millimeter-wave frequencies are prohibitively costly to all but a limited number of users.
Further, present FPIAs offer a limited imaging throughput capability because they employ a slow, mechanical calibration source for system gain adjustments, a critical task which must be performed frequently to minimize systematic errors in the FPIA pixels due to gain drifts. Presently, calibration of the FPIA pixels by mechanical means may take anywhere from several seconds to several minutes. Still further, the mechanical calibration source adds significantly to the size and weight of the imaging device.
In light of the above discussion, it has been considered very desirable to develop new FPIA designs for use in direct detection microwave or millimeter-wave radiometric imaging devices which overcome the above disadvantages associated with the current technology.